The invention relates to an alumina ceramic and, more particularly, to a ceramic made of alumina with a low alkali metal oxide content for heating conductors. Ceramic components with integrated heating conductors in the form of printed patterns consisting of a cermet are endangered by electrochemical decomposition of the carrier ceramic, of the ceramic cover layer and/or of the ceramic supporting structure. The electrolytic decomposition is produced by ionic conduction under the applied heating voltage, which is, for example, below 24 V direct-current voltage in the case of motor-vehicle applications, and can finally lead to the destruction of the heating conductor on account of the separation of cations with alloy formation and/or by separation of oxygen with oxide formation or by crack formation on account of the excess oxygen pressure arising below the insulating cover layer. This applies all the more the higher is the application temperature, and can become critical at temperatures as low as 600.degree. C. Such ceramic components with integrated heating conductors are used, for example, in sheathed-element glow plugs, in lambda probes or similar electrochemical sensors, in heated nozzles for various intended applications and in continuous-flow heaters for boilers or coffee machines in the form of small plate-shaped, pin-shaped or tubular heaters.
Aluminium oxide, as it is available in the trade, normally contains up to 0.5% by weight sodium oxide and/or potassium oxide. Usually, the aluminium oxide is used in this form, especially in mass-produced products, because the purer specifications of the aluminium oxide are far more expensive, as compared with those mentioned.